1. Field of the Invention
The present invention relates to passive target location and more specifically to the location of targets by determining the time difference of arrival of electromagnetic emissions between a master station and two stations linearly positioned on either side thereof.
2. Description of the Prior Art
Early in the history of the radar art passive systems, e.g. systems that utilize target generated transmissions as signal sources, were employed to determine the location of radiating sources. Angles to the radiating sources at the two locations, a known distance apart, were determined with receiving antennas having cardioid patterns. The angle to the source being determined when the apex of the cusp of the cardioid was in the direction of the radiating source. The two measured angles and the known distance between the antenna locations were then triangulated to establish the position of the target. Since the angular range of the cardioid pattern cusp were relatively broad, inaccurate angular measurements resulted giving rise to relatively imprecise target location determinations.
Additionally, these systems were operative only in a single source environment. If two or more sources at different locations were emitting signals within the frequency reception band of the system, the system performance was seriously impaired.
Improved accuracy was realized with the advent of monopulse receiving systems. These systems provided significant improvements in the measurement of the angle to the radiated source, thus permitting a more accurate determination of the target location. In a multiple simultaneous signal environment operating within a monopulse beamwidth and within the bandwidth of the system, however, monopulse systems will provide an indication of a single target at a location determined by the angular centroid of the radiating sources. Thus these systems provide useful information of the location of emitters radiating time overlapping signals and operating within the receiver bandwidth only when one emitter is within the monopulse beamwidth. Though monopulse systems may provide target location information when the received signals are time separated, target location ambiguities exist when only two receivers are employed on a baseline.
In addition to the multiple target limitations, prior art systems do not have broadband capabilities and operate only within relatively narrow frequencies band.